This invention relates to a method and instrument design for detecting small inclusions in solid media such as transparent sheets of glassy materials and plastics. Bulk glass such as high purity fused silica (HPFS(copyright)) is another example of solid media.
Detecting small (micron and submicron) inclusions in glass always has been a challenge. The difficulties associated with various practices are sensitivity, resolution, depth of focus, to name a few. Microscopy has the capability to detect inclusions down to the submicron range, yet it has an extremely narrow depth of focus and a small sampling area at high magnification. These are necessary for detecting small inclusions. If used alone, these restrictions make it next to impossible to analyze bulk glass. Diffused reflection/scattering has been used to identify inclusions. After mapping their location, the inclusion can be further determined by microscopy. Nevertheless, the detection limit for the diffused reflection/scattering approach is about 5 microns and as low as 1 xcexcm. In addition, the thickness of the glass is again somewhat restricted by the narrow depth of focus of the microscopy technique.
Small particles suspended in a fluid media, such as a liquid or gas, on the other hand, can be measured routinely by light scattering techniques. The differences between inclusions in a solid glass and particles suspended in a fluid are critical. One difference is that an inclusion in a glass is stationary. Its concentration level is normally very low, thus the signal intensity is so weak that it can hardly be distinguished from noise. Noise is the cross talk between surface detection (surface signals) and in depth detection (internal signals). In addition, the location of inclusions in glass would be valuable information. Due to the dynamic nature of the suspended particles in a fluid media, their location cannot to mapped. As a result, current existing instruments are not designed with particle location mapping capability. Nevertheless, we have found that the principle behind the measurement of particles suspended in fluid media is applicable for measurement of inclusions in solid glass.
Our method and instrument design adapts the principle of light scattering to directly measure inclusion in a solid media. We redesigned the instrument intended for size measurement of particles dispersed in fluid media to achieve detection of inclusions in bulk glass. We now can map the location of inclusions in glass. The outcome is directly applicable to inclusion detection for HPFS(copyright) photomask and LCD glass.
Our light scattering technique for size measurement is based on the fact that an illuminated particle (or inclusion) serves as a secondary radiation source in a manner which is related to its size. When illuminated with a beam of monochromatic light using a laser beam as the primary light source the angular distribution of the scattered intensity originated from the inclusion in the micron to submicron range, is a function of the following: the angular distribution of the scattered intensity is a function of scattered light and the incident beam, the wavelength of the incident light, and the index of refraction of the particle relative to that of the surrounding media.